Reaction resin mixtures hardenable using ultraviolet (UV) light in accordance with a cationic mechanism are becoming increasingly important technically, because they yield molded materials having excellent thermal-mechanical properties. The chemical basis for these reaction resin mixtures are compounds which contain oxirane rings, such as epoxy resins and/or vinyl ethers. Whereas reaction resin mixtures on the basis of vinyl ethers are distinguished by their rapid speed of reaction (in hardening), reaction resin mixtures on the basis of epoxides prove to be advantageous due to a more favorable performance with respect to shrinkage in the hardening process.
In the hardening process of reaction resin mixtures, the UV light is absorbed by a photoinitiator, which, as a result of subsequent reactions, forms carbocations or protons; these are the actual active species for the start of the polymerization. Conventional photoinitiators for cationic polymerization are, for example, triarylsulfonium salts. Although they possess good reactivity in response to UV irradiation, they are thermally (i.e., in response to an increase in the temperature) very stable and therefore incapable of thermally initiating the cationic polymerization.
Thermal hardening is always necessary when it is impossible to irradiate all areas of the resin. For example, this is the case when thicker layers are to be produced or when light-diffusing or light-absorbing additives, such as fillers, pigments and dyes are contained in the reaction resin mixture. In those cases, the light is highly absorbed or diffused in the layer regions closer to the surface, so that the light transmitted into the deeper layer regions is not sufficient to bring about a (complete) hardening.
In addition, a UV hardening is impossible when, in accordance with the method, areas are present which are not accessible to direct irradiation. In gluing together non-transparent joining parts as well as electronic components and assemblies, the adhesive agent is applied first, and then the component part is mounted. By irradiation using UV light, in this context, only those edge areas can be hardened where the adhesive agent oozes out; underneath the component part, the hardening must be brought about through an additional process, e.g., thermally induced. The situation is comparable when components and assemblies, for protection against environmental influences, are provided with a protective lacquer. For due to capillary action, the lacquer migrates under the component parts, and there, once again, it cannot be hardened by irradiation.
Reaction resin mixtures that are hardenable by UV light can be used for the production of complex plastic models by stereo lithography with the aid of 3D CAD data. In this context, the surface of a liquid photopolymer, hardenable by laser light, is irradiated patternwise by a computer-controlled laser beam, a first layer of the three-dimensional structure to be produced being cured. Subsequently, this layer is coated with fresh photopolymer and is once again irradiated patternwise by the laser. In this way, a second hardened layer of the three-dimensional structure arises, joining with the first one. This process continues until the entire structure is produced, which, in this context, expands into the photopolymer bath. The "green part" formed in this way and only partly cured, is subsequently cured to a great extent as a result of longer irradiation by UV-A light.
In this method, photopolymers based on epoxy resins are advantageously used, the photopolymers demonstrating a more favorable performance with respect to shrinkage in comparison to polymers based on acrylate, so that greater dimensional and form stability can be achieved. In addition, a second cure of the partly hardened green parts is also possible by raising the temperature. This thermal hardening is successful only when all the areas to be strengthened were previously irradiated. Purely thermal hardening, on the other hand, is impossible in conventional photopolymers using triarylsulfonium salts.
However, the possibility of a thermal hardening is a goal because, in the case of a partial irradiation, for example for producing contours and grids, considerable time can be saved in comparison to an all-over irradiation. The assumption, in this context, however, is that the non-irradiated areas can be cured by a subsequent thermal process, because otherwise molded-material properties are only achieved to an inadequate degree. A second irradiation of these areas in the interior of thicker parts is impossible because the light will be absorbed by the photoinitiator in the edge layers and thus is not available for a photochemical process in the interior of the parts.
The speed of hardening in the cationic polymerization of epoxy resins is slower than in polymerization of acrylates, and also a higher UV dose is required for the hardening. Therefore, the most powerful UV lasers possible must be used. These are available, for example, using argon-ion lasers (having wavelengths of 351 and 364 nm) as well as using frequency-tripled Nd:YAG lasers (having a wavelength of 351 nm), but in these wavelengths, the absorption capacity of the triarylsulfonium salts, which are usually employed, is too slight to be able to effectively form cations or protons.
From European Patent Application No. 0 370 693 A2, it is known to use as photoinitiators onium salts, which form a Br.o slashed.nsted acid under irradiation with visible light. These are sulfonium, arsonium, ammonium, and phosphonium salts (having an S, As, N or P atom), which contain a chromophore which absorbs visible light, the chromophore being separated from the S, As, N or P atom by an isolating group, which prevents a n-resonance (between the chromophore and the other substituents). The onium salts additionally contain at least one substituent which represents an electron-attracting grouping and has an unoccupied molecular orbital at a lower energy level than the light-absorbing chromophore. Examples of the onium salts of this type are: 4-cyanobenzyl-2-[5-naphthacenyl] benzylphenyl-sulfonium hexafluorophosphate and -trifluoromethane sulfonate as well as phenyl-p-cyanobenzyl-4-[6,7-dimethoxycumarin-methyl]sulfonium hexafluorophosphate and -trifluoromethane sulfonate. Activation of the onium salts by UV light does not take place, nor does a purely thermal formation, i.e., without previous irradiation, of the Br.o slashed.nsted acids.